Ultrasound is widely used to guide the placement of interventional instruments such as needles to targets in the human body. With this imaging modality, it is possible to generate two- and three-dimensional images that provide anatomical information relevant to target identification. A typical ultrasound system can utilize a transducer array to deliver acoustic pulses into the body and to temporally resolve reflected acoustic pulses. A typical ultrasound system provides a two-dimensional image that derives from a scan plane within tissue.
One of the challenges in ultrasound-guided percutaneous interventions is the visualization of a needle tip. During an insertion, the needle tip can readily stray from the image (scan) plane so that its position relative to the ultrasound image is unclear. Reorientation of the imaging transducer of the ultrasound system and/or reinsertion of the needle to bring the tip back into the image plane can be time-consuming and cause patient discomfort.
Another potential problem with locating instruments during ultrasound imaging is that an instrument may have a smooth surface, so that acoustic pulses are specularly reflected from the instrument surface in a direction away from the transducer surface, thereby preventing the instrument from being visible on the ultrasound image. One solution is to use echogenic needles that increase the range of angles at which acoustic pulses are reflected from the needle surface, which may include indentations on the needle cannula and stylet [see, e.g., U.S. Pat. No. 5,490,521] or polymer coatings with microbubbles [see, e.g., US2005-0074406]. Companies that supply needles that have coatings or surface modifications to increase the echogenicity of the needles so that they are more prominent in ultrasound images include Cook Medical (www.cookmedical.com), B Braun (www.bbraun.co.uk) and Pajunk (www.pajunk.com). However, echogenic needs are only visible when they are in the ultrasound imaging plane. In a recent study of needle visibility, commonly-used echogenic needles were not visible on the ultrasound image during 45% of the procedure time [Hebard S and Hocking G. Reg. Anesth. Pain Med. 2011; 36:185-189]. Echogenic needles may also introduce large artifacts in ultrasound images that risk obscuring anatomical detail.
Another solution is to mechanically vibrate the instrument so that it can be detected with Doppler ultrasound, as has been suggested for the case of a biopsy needle [see, e.g., U.S. Pat. Nos. 5,095,910 and 5,425,370]. However, this solution again has the limitation that the instrument typically cannot be visualized when it is outside the scan plane.
Commercially available mechanical guides such as those developed by Civco (www.civco.com), or by Bard Access Systems (see www.bardaccess.com) for the Site-Rite® Ultrasound system, mechanically constrain the trajectory of needles. These are generally provided as removable accessories to ultrasound imaging probes, and are designed to limit the direction in which the needle is inserted, so that the needle is maintained close to the scan plane, or at least the needle is more frequently in the ultrasound imaging plane. Accordingly, visibility of the needle in the ultrasound image should be improved. However, once the needle is secured in the mechanical needle guide, approaches to the target cannot be changed without complete withdrawal and reinsertion of the needle. As a result, mechanical needle guides are unsuited to most anaesthesia and interventional pain management procedures, where fine adjustments in needle trajectory and depth are required to achieve adequate local anaesthetic spread around the target nerve. Furthermore, a needle may bend as it passes through tissue, and therefore may still follow a trajectory that lies outside the ultrasound scan plane.
It has also been suggested that the scan plane could be chosen automatically to maximize the visibility of an instrument. For example, U.S. Pat. No. 6,524,247 discloses that the ultrasound beam could be adaptively tilted, while U.S. Pat. No. 6,764,449 discloses that two-dimensional images could automatically be extracted from three-dimensional image volumes in such a way that the needle visibility is maximized However, these two approaches both have the disadvantage that they are typically dependent on robust, real-time segmentation of images to identify instruments. Similarly, devices from Sonosite (www.sonosite.com) use software enhancements of an ultrasound imaging system to implement image processing and beam steering techniques in order to increase the visibility of needles. Again however, these enhancements are only relevant when a needle is the ultrasound imaging plane. Furthermore, the positions of needle tips in the body are not explicitly determined, and it is also difficult to use this approach with devices having low echogenicity such as catheters.
Another challenge associated with locating an instrument during ultrasound imaging is that there can be a very low difference between the acoustic impedance of the instrument and the tissue surrounding the instrument. One solution to this problem is based on photoacoustic time-of-flight localization. For example, U.S. Pat. No. 7,068,867 discloses a system in which acoustic waves are generated by the instrument or in tissue adjacent to the instrument by means of the delivery of pulsed light and the photoacoustic effect. In this system, acoustic waves generated by the absorption of pulsed light are received by the ultrasound imaging transducer, and time-of-flight measurements then allow for instrument localization However, the lasers that are currently employed in such a system are expensive. Furthermore, having lasers deliver pulsed light out of instruments may be problematic with respect to eye safety in a clinical environment.
Instrument localization can also be performed with markers positioned on the instrument that are tracked by external sensors, for example, by optical and/or electromagnetic (EM) tracking [see, e.g., Glossop et al., The Journal of Bone and Joint Surgery, 91:23-28 (2009)]. Similarly, Ultrasonix and GE provide EM tracking, whereby sensors in the needle and ultrasound imaging probe are tracked by an external field generator that is positioned close to the patient. However, such sensors are generally expensive and are currently not disposable. Furthermore, the external field generator is typically bulky, likewise the sensors are typically bulky (and may therefore be incompatible with small needles). In addition, marker-based localization systems may involve long set-up times and calibration procedures, such as to integrate a non-disposable sensor into a disposable needle component, which makes them unattractive for short procedures. In addition, such systems may be sensitive to subtle changes in the external environment, for example, the introduction of metal objects (e.g. a surgical tool) that alter EM fields in the case of EM tracking and greatly reduce tracking accuracy, or opaque objects that affect line-of-sight in the case of optical tracking.
Various techniques have been proposed to identify the position of a medical needle during percutaneous interventions by receiving acoustic waves generated by the imaging transducer with a second transducer integrated into the needle. U.S. Pat. No. 5,158,088 proposes that a transducer positioned at the needle tip could receive acoustic pulses transmitted by an imaging transducer, thereby allowing for an alert to be provided to the physician when the needle tip is in the scan plane. This device has the limitation that it generally does not provide information about the position of the needle tip when the needle tip is not in the scan plane. U.S. Pat. No. 4,249,539 and U.S. Pat. No. 5,161,536 propose that needle tip localization is performed by measuring the time-of-flight of individual spatially-focused acoustic pulses delivered from an imaging transducer to a second transducer positioned at the needle tip. In the case of U.S. Pat. No. 4,249,539 the needle transducer confirms receipt of the ultrasound pulse by transmitting its own ultrasound pulse back to the imaging transducer. However, such an approach has the disadvantage that acoustic pulses from the imaging transducer are typically only received by the needle transducer when the needle tip intersects the scan plane. Nikolov and Jansen have demonstrated needle localization in two and three dimensions with time-of-flight measurements of individual unfocused acoustic pulses [see, e.g., J. Nikolov and J. Jansen, Ultrasonics Symposium, 2008. IUS 2008. IEEE, pp. 479-482 (2008)]. This publication describes a proposed transmission of a single pulse and subsequent reception of said pulse, followed by transmission of a second single pulse and subsequent reception of said pulse, and so on. One significant disadvantage of this solution is that the process of emitting and receiving a large number of individual pulses, which the authors suggest is useful for reducing errors, could be very time-consuming. A second disadvantage is that it involves an ultrasound imaging probe which is capable of providing three-dimensional ultrasound images—however, such devices are currently bulky and prohibitively expensive for many ultrasound-guided procedures. WO 2011/138698, U.S. Pat. No. 6,587,709 and WO 2012/066437 also disclose medical device tracking based on transmission of acoustic pulses between a catheter and a 3D imaging transducer. However, these proposals are also dependent on the presence of an ultrasound imaging probe that is capable of providing three-dimensional ultrasound images, whereas this type of probe is not available for a wide range of medical procedures.
WO 98/39669 discloses an ultrasound imaging head having a window through which ultrasound is transmitted and received by an image transducer. The imaging head also holds three or more position transducers that form a plane perpendicular to the ultrasound imaging beam. There are also reference transducers mounted to the patient's body. The orientation of the imaging plane with respect to the coordinate system defined by the reference transducers can be calculated by determining the location of the position transducers on the imaging head. This then allows a real-time imaging output to display in three-dimensions the position of an instrument relative to the ultrasound imaging plane.
Despite the range of existing solutions discussed above, the problem of accurately and consistently locating instruments during ultrasound imaging in a manner that is compatible with most clinical procedures remains Consequently, procedures may involve multiple instrument insertions that increase patient discomfort and procedure duration, and may result in additional risks such as the inadvertent penetration of an important tissue structure when the position of the instrument tip is not known. Accordingly, there is a significant need for a system that can determine the position of an instrument accurately and in real-time, with minimal compromise to scanning speed.